In recent years, optical windows for near infrared light have received a great deal of attention for optical communication and other special applications. Such windows must be transparent to near-infrared light (typically with wavelength in the range of 800-1600 nm). At the same time, the windows should be capable of shielding against the transmission of light which may adversely affect the sensors behind the windows. Besides, the optical windows should be stable under different ambient conditions, and capable of resisting humidity and high temperature. The present known optical filters include absorption filters, diffraction filters generated by optical holographic techniques, and transmittive or reflective multiple layer (or multilayer) metal-dielectric or all dielectric filters as disclosed in U.S. Pat. Nos. 5,119,232, 5,007,689 and 4,865,405.
Absorption filters are usually composed of material which possesses intrinsic absorption in the desired optical range or material which is added with dye for absorption in the desired optical range. The drawback of these materials is the degradation of the performance of the filters due to the absorption of energy.
Diffraction filters generated by optical holographic techniques utilize photosensitive gelatine materials. However, such gelatine can easily be affected by ambient conditions, particularly humidity and heat. Such filters can only be used for low power light source or laser source.
Multilayered dielectric transmission or reflection filters are basically composed of alternating layers of materials with high refraction index and low refraction index deposited on a substrate. Due to limitation of the material, an all dielectric multilayered near-infrared optical window requires approximately 60 layers. Such techniques are described in the following articles: (1) J. A. Dobrowolski, "Coating and Filters", in Handbook of Optics, W. G. Driscoll, ed., McGraw-Hill Book Co., New York, 1978, Chapter 8; (2) J. A. Dobrowolski, J. R. Pekelsky and A. J. Waldorf, "Practical Megnetron Sputtering System for the Deposition of Optical Multilayer Coatings", Appl. Opt., 31, pp.3784-3789 (1992). From the design and fabrication standpoint, such a structure is very difficult to realize. In addition, due to the large number of interfaces between the multilayers, there are many existing defects which can cause undesirable scattering or thermal damage as a result of extra light absorption and peeling of the layers.